How to emit real random particles
You can use Drivers to get "random" Seed in Particles settings.


  As @Crantisz pointed you can just paste #int( 99 * noise.random() )
  as a Seed value and Driver will be automatically added.


Long way:


Right click on Seed and Add Driver > Manually Create Later (Single).
Go to Graph Editor and switch to Drivers mode.
In Drivers tab paste this expression: int( 99 * noise.random() )
Update Dependencies.


Now every time you open Blender Seed value will be different.